Fluororubbers have exceptional heat resistance and chemical resistance as compared with other rubbers, and therefore they are used as materials of O-rings, gaskets, packing and the like in various industrial fields, including the automobile industry.
Examples of vulcanization processes of fluororubbers include organic peroxide vulcanization, polyhydroxy vulcanization (polyol vulcanization) and polyamine vulcanization. In recent years, the fluororubbers are almost always used after they have been subjected to organic peroxide vulcanization. This provided an improvement in crosslinking efficiency and improvement in chemical resistance by the formation of a chemically stable C--C bond at the crosslink site. However, the fluororubbers having been subjected to organic peroxide vulcanization are not always satisfactory in the compression set when compared with fluororubbers having been subjected to polyhydroxy vulcanization.
Fluororubbers are generally compounded with acid acceptors to neutralize acid substances produced in the vulcanization process and to prevent corrosion of the counterparts caused by the fluororubber products. Examples of the acid acceptors include lead monoxide (PbO, litharge), zinc white (ZnO), calcium hydroxide and magnesium oxide. Fluororubbers compounded with lead monoxide (PbO, litharge) have an expectation of being improved in compression set, heat aging properties, steam resistance and chemical resistance, but disposal (for land reclamation) of such rubbers has recently become difficult in view of the environmental problems.
When zinc white (ZnO), calcium hydroxide and magnesium oxide are used as the acid acceptors in place of lead monoxide, the resulting vulcanized fluororubbers are all inferior to the vulcanized fluororubbers obtained by the use of lead monoxide in compression set, heat aging properties and steam resistance.
The present inventors have earnestly studied the problems mentioned above, and as a result, they have found that when a fluororubber is compounded with hydrotalcite, the resulting fluororubber is remarkably improved in compression set and other properties even if the fluororubber is subjected to organic peroxide vulcanization.
The present inventors have further found that especially when the fluororubber is a tetrafluoroethylene/perfluoromethyl vinyl ether copolymer, a vulcanized product is produced which is a high value added material, having excellent compression set, high chemical resistance, relatively low toxicity and maintenance-free properties. Based on this finding, the present invention has been accomplished.
In Japanese Patent Publication No. 57125/1989, the present applicant proposed a process for preparing a peroxide-vulcanizable fluorine-containing elastomer, comprising homopolymerizing or copolymerizing a fluorine-containing olefin of 2 to 8 carbon atoms in the presence of a compound containing iodine and bromine represented by the formula RBr.sub.n I.sub.m (R is a fluorohydrocarbon group, a chlorofluorohydrocarbon group, a chlorohydrocarbon group or a hydrocarbon group, and n and m are each 1 or 2). This publication describes that not only the organic peroxides and the polyfunctional unsaturated compounds serving as co-crosslinking agents but also oxides or hydroxides of divalent metals (e.g., oxides or hydroxides of calcium, magnesium, lead and zinc) serving as crosslinking assistants are employable in the peroxide vulcanization of the fluorine-containing elastomer, and also describes that these crosslinking assistants serve also as the acid acceptors.
Japanese Patent Laid-Open Publication No. 88076/1998, describes that, for the direct vulcanization adhesion between a layer made of either a fluororubber or a fluororesin and a layer made of an epichlorohydrin rubber, a composition comprising (A) an epichlorohydrin rubber, (B) a thiourea compound, (C) sulfur, (D) an aromatic disulfide compound and (E) 1,8-diazabicyclo[5.4.0]undecene-7 or its weak acid salt in a prescribed quantity ratio is used as a material for forming the epichlorohydrin rubber layer.
In the above publication, it is also described that the composition can be further blended with (F) a hydrotalcite compound, specifically, DHT-4A available from Kyowa Kagaku K.K., and that by virtue of blending the composition with the compound (F), adhesion between the two layers can be improved, the compression set, storage stability and sour gasoline resistance of the epichlorohydrin rubber layer can be improved, and the problem of toxicity can be solved because no lead compound is used as the acid acceptor.
Japanese Patent Laid-Open Publication No. 139970/1998, describes a fluororubber composition comprising a vinylidene fluoride/perfluoro (methyl vinyl ether)/tetrafluoroethylene terpolymer having a fluorine content of not less than 66%, a liquid fluororubber, carbon black having an iodine adsorption of about 40 to 100 mg/g and a hydrotalcite/calcium hydroxide mixture having a mixing ratio by weight of about 40/60 to 95/5. In this publication, it is also described that the composition is favorable as a vulcanization molding material of seal rings for FFV (Flexible Fuel Vehicle) fuel oil and can provide seal rings which are excellent in fuel oil swell resistance, low temperature properties, amine cracking resistance and metal corrosion resistance. It is further described that, in the preparation of the fluororubber composition, a vulcanizing agent such as an organic peroxide, a co-crosslinking agent which is a polyfunctional unsaturated compound, a vulcanization accelerator, a processing aid, etc. are optionally added to the above components and kneaded, and the kneadate is subjected to vulcanization molding.
None of the above publications, however, teaches or suggests that vulcanization of a vulcanizable fluororubber composition comprising tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer, an organic peroxide, a co-crosslinking agent and hydrotalcite makes it possible to obtain a vulcanized product having excellent compression set and which causes no environmental pollution even when discarded.
The present invention solves the problems found in the prior art as described above, and provides a tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer composition having excellent compression set and air heat aging properties and causing no environmental pollution even when discarded.